1) Field of the Invention
The present invention relates to a wavelength selector switch that selects wavelength-multiplexed light according to wavelengths and outputs the selected light to a desired port. The present invention, particularly, relates to the wavelength selector switch which allows all-optical cross-connect.
2) Description of the Related Art
Optical networks, which use wavelength-division multiplexing (WDM) communication, have been progressing rapidly to allow increasing traffic due to spread of the Internet and to make use of the existing optical fiber networks. The WDM communication, which is applied to a point-to-point network at present, has been researched for application to a ring network and a mesh network. Such a network allows optical processing in its network node with an optical add/drop multiplexer (OADM), which divides and combines light of desired wavelength, and an optical cross connect (OXC), which does not need opto-electric conversion. This optical processing can lead to control (e.g. setting and cancellation) of a dynamic path based on wavelength information. Progress of a photonic network technology which uses such optical technology fully is disclosed in, for example, U.S. Pat. No. 6,204,946.
The OADM or OXC includes a wavelength selector switch. FIG. 34 is a schematic diagram of the wavelength selector switch. As shown in FIG. 34, optical transmission paths of two system, i.e. a first optical transmission path 111a is operated as a main line of an optical fiber transmission linear path and a second optical transmission path 111b is operated as a branched line.
A wavelength selector switch 110 is disposed between the first optical transmission path 111a and the second optical transmission path 111b such that the wavelength selector switch 110 connects nodes of the first optical transmission 111a and the second optical transmission path 111b. The wavelength selector switch 110 includes two optical input ports In and Add and two optical output ports Pass and Drop. Concretely, the ports are named as an input port In, a combining port Add, a passing optical output port Pass, and a dividing optical output port Drop respectively.
Light that is input via the optical input ports In and Add, is a WDM signal that includes a plurality of wavelength components. The typical wavelength space is 100 GHz (0.8 nm) and a number of wavelengths is from a few wavelengths to a few tens of wavelengths (for example 32 wavelengths; in this case, for λn, n=32). A circulator 112a is combined with (inserted into) the node of the first optical transmission path 111a and a circulator 112b is combined with (inserted into) the node of the second optical transmission path 111b. The circulators 112a and 112b have a function of outputting light that is input to an optical input port C1 from an optical input-output port C2 and a function of outputting light that is input to the optical input-output port C2 from an optical output port C3 respectively.
An optical switch module 114 is disposed between optical input-output ports C2 of the circulators 112a and 112b. The optical switch module 114 includes optical systems 117, a diffraction grating 118, and a micro mirror array 121 formed by micro electromechanical systems (MEMS). The optical systems 117 (not shown in detail in the diagram) includes lenses like a collimating lens and a collective lens (Refer to U.S. Pat. No. 6,204,946).
According to such structure, light that is output from the optical input-output port C2 of the circulator 112a is incident on a grating surface of the diffraction grating 118 via the optical system 117. The diffraction grating 118 is an element that imparts a different diffraction angle to a different wavelength component that is incident at a predetermined angle. Therefore, a WDM signal that is reflected from the diffraction grating 118 is separated spatially for every component of wavelength λ and is collected on the micro mirror array 121.
By controlling a micro mirror 122 for different wavelengths that are provided in the micro mirror array 121, the light of wavelengths from λ1 to λn that is collected on the micro mirror array 121 is either reflected through the same optical path as that of the incident light or is reflected through a different optical path.
The micro mirror array 121 is a mirror that is manufactured by using a micro machine technology. The micro machine technology is disclosed in pages 94 to 103 of February 2002 issue of a Journal of IEICE (The Institute of Electronics, Information & Communication engineers) and “Micro mechanical optical device” on pages 1274 to 1284 of no. 11, 69th volume of JAPANESE JOURNAL OF APPLIED PHYSICS (published by The Japan Society of Applied Physics). The micro mirror 122 as shown in the diagram is an arrangement of a plurality of micro mirrors 122a to 122n that are arranged at a distance of tens of μm from each other. The number of micro mirrors 122n is same as the number n in which wavelengths λ is split (separated) and one micro mirror corresponds to one wavelength component. Light of wavelengths from λ1 to λn that is separated by the diffraction grating 118 is incident on the micro mirror 122 (122a to 122n) in a position corresponding to that particular wavelength.
FIG. 35 is a side view of a micro mirror arrangement that is provided in the micro mirror array. FIG. 36 is an illustration of an operation of the micro mirror. The micro mirror 122 includes a substrate 123, a support 124 that protrudes from the substrate 123, and a reflector 125 that is supported at a center by the support 124. Surface 125a of the reflector 125 is a total reflecting surface from which light A and B are reflected totally. The substrate 123 includes a pair of electrodes 126a and 126b in the form of a flat plate facing the reflector 125 with the support 124 sandwiched between the electrodes 126a, 126b and the reflector 125. An electrode 127 in the form of a flat plate is provided an overall rear surface of the reflector 125 facing the electrodes 126a and 126b. 
When voltage is applied to the electrode 126a, static electricity is generated between the electrodes 126a and 127 that are facing each other. Due to the static electricity, the electrode 127 is attracted towards the electrode 126a and the reflector 125 is inclined to one side with the support 124 as a center, as shown in FIG. 35.
With the electrode 127 attracted towards the electrode 126a, light incident on the reflector 125 is allowed to be reflected in a direction same as that of the light beam A. Concretely, the surface 125a of the reflector 125 is adjusted such that the surface 125a is orthogonal (at right angles) to direction of light A that is incident. Due to this, the light that is input from the optical input port In can be returned in the same optical path and can be output from the optical output port Pass.
On the other, when voltage is applied to the electrode 126b, static electricity is generated between the electrodes 126b and 127 that are facing each other. Due to the static electricity, the electrode 127 is attracted towards the electrode 126b and the reflector 125 is inclined to one side with the support 124 as the center as shown in FIG. 18.
With the electrode 127 attracted towards the electrode 126b, light beams A and B that are incident on the reflector 125 are reflected to follow different optical paths. Concretely, the reflector 125 is adjusted such that the angle of the surface of the reflector 125 is at predetermined angles (θ) with the light beams A and B that are incident. This enables to output the light beam A that is input from the optical input port In from the optical output port Drop by switching to an optical path of the other light beam B. Similarly, the light beam B that is input from the optical input port Add can be output from the optical output port Pass by switching to an optical path of the other light beam A.
Thus, the light input to the optical input ports In and Add can be selected according to wavelengths λ1 to λn and can be output from the optical output ports Pass and Drop upon switching. For example, as shown in FIG. 34, let the light from the (optical input port) In of the first optical transmission path 111a has wavelengths λ1, λ2, and λ3 and the light from the (optical input port) Add of the second optical transmission path 111b has wavelengths λa, λb, and λc (where wavelengths λ1 =λa, λ2=λb, and λ3=λc). A certain wavelength can be selected and be made to switch to a different optical path by changing the angle of the micro mirror 122. Thus, the wavelengths λa, λ2, and λc can be output from the optical output port Pass of the first optical transmission path and the wavelengths λ1, λb, and λ3 can be output from the optical output port Drop of the second optical transmission path.
Thus, by using the micro mirror array 121, the direction of reflection of the light beams A and B of wavelengths λ1 to λn that are incident on the micro mirror array 121 can be switched for each frequency component. Thus, it is possible to use the second optical transmission path (branched line) as a back-up circuit of the first optical transmission path (main line) and to perform operations like transmitting by switching a specific wavelength λ only, for which the transmission was hindered in one of the optical transmission paths, to the other optical transmission path.
However, in the wavelength selector switch 110 that employs the micro mirror array, the all-optical cross-connect could not be achieved. The micro mirror 122 that is used in the micro mirror array 121 can switch the light from the second optical input ports In and Add and the second optical output ports Pass and Drop mutually when the angle is changed as shown in FIG. 36. Whereas, in a situation that is illustrated in FIG. 35, light incident from one of the optical input ports In of the first optical transmission path cannot be returned to the optical output port Pass.
In other words, in a situation that is illustrated in FIG. 35, it is not possible to switch an overall optical path of the other optical input port Add of the second optical transmission path 111b. In this situation, even if the light is incident from the optical input port Add, the micro mirror 122 is inclined at an angle such that the incident light cannot be reflected to any of the ports. Thus, the conventional wavelength selector switch 110, due to the arrangement in the micro mirror array 121 has not been able to achieve the all-optical cross-connect in which the light from the two optical input ports is always switched to any of the two optical output ports. An all-optical cross-connect at present implies a possibility of switching the light to a desired port for each wavelength by using a structure of 2×2 ports (i.e. two input ports and two output ports).
The voltage is to be applied continuously to either the electrode 126a or the electrode 126b to maintain the status in which the angle of the micro mirror 122 in the micro mirror array 121 is changed. If the voltage is stopped, the switching of the optical path that is maintained cannot be continued. Apart from this, an optical axis of the light incident on and output from the micro mirror 122 from the plurality of micro mirrors in the micro mirror 122 has to be adjusted which is a tedious job. Moreover, the components in the arrangement being the micro members, the component cost goes high and the durability of the structure that changes the angle of the micro mirrors 122 becomes an issue.